Lithographic apparatus and associated method

A method of determining compatibility of a patterning device with a lithographic apparatus, wherein the lithographic apparatus comprises:

• a collector to collect radiation emitted by a plasma to provide a radiation beam,
• an illumination system configured to receive and condition the radiation beam to form a conditioned radiation beam, and
• a projection system configured to image a patterned radiation beam to an image plane, wherein the patterned radiation beam is formed by the patterning device configured to impart the conditioned radiation beam with a pattern in its cross-section to form the patterned radiation beam,
• the method comprising:
• determining an intensity distribution of the radiation beam;
• using the determined intensity distribution to calculate a non-uniformity or change of non-uniformity of intensity caused by contamination and/or degradation of the collector;
• determining the effect of the non-uniformity or the change of non-uniformity on a characteristic of the image of the patterned radiation beam;
• and
• determining the compatibility of the patterning device with the lithographic apparatus based on the effect of the non-uniformity or change of non-uniformity on the characteristic.